Process and apparatus for resin impregnation of a fibrous substrate

ABSTRACT

Apparatus and process are described for impregnating a fibrous substrate with a thermosettable resin. The process involves the use of resin application means comprising a moving surface on which is positioned a liquid-form thermosettable resin in essentially uncured state, passing a fibrous web in countercurrent contact with the thermosettable resin so as to transfer the thermosettable resin into the fibrous web, and passing the resin-containing fibrous web to a heating zone to partially cure the resin and form a prepreg. The process is particularly suited for application of a solventless resin formulation to a glass web in the preparation of a prepreg for an electrical laminate. The apparatus includes resin application means comprising a movable surface; means for applying a liquid-form thermosettable resin onto the movable surface; means for advancing, in a countercurrent direction with respect to the direction of motion of the movable surface, a fibrous web to the movable surface and in contact with the thermosettable resin thereon and thence to a resin cure zone; and means not in contact with the opposite side of the fibrous web at the point of resin transfer for maintaining tension in the glass web sufficient to promote transfer of the liquid resin film from the movable surface into the interior of the glass web.

This is a continuation of application Ser. No. 07/982,264, filed Nov.25, 1992, which is a continuation of application Ser. No. 07/796,882,filed Nov. 25, 1991, which is a continuation of application Ser. No.07/583,119, filed Sep. 17, 1990, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the preparation of fiber-reinforcedthermosettable resin articles. In a specific embodiment, the inventionrelates to a process and apparatus for impregnation of a glass substratewith a solventless thermosettable resin system.

The manufacture of the cured thermosettable resin base of an electroniccircuit board begins with the impregnation of a fibrous glass substratewith a liquid thermosettable resin system. The resin-impregnatedsubstrate is then partially cured to form a "prepreg." A set of layeredprepregs is then heated under pressure to fully cure the resin and toform a hard laminate, which serves as the base for electric circuitry.

Although there exist thermosettable resins, such as low molecular weightepoxy resins, which are liquid at room temperature, current circuitboard requirements make it necessary to use high-performance resinssystems which are solids or viscous liquids at room temperature and toapply the resins to the substrate in melt or solution form. Attempts toprocess thermosettable resins in the melt, however, have not beensuccessful because of the difficulty of achieving good "wet-out," orsaturation of the fiber by the resin, and also because the hightemperatures necessary to melt the resin cause the resin to cureprematurely, further adding to the wet-out problem.

Current commercial processes for preparing prepregs apply the resin tothe substrate using an organic solution of the resin. Solution processesmust include a step, usually carried out in conjunction with partialcuring of the resin, in which the solvent is removed from the prepreg byheating the solvent to its volatilization temperature. Such a processhas a number of disadvantages: First, it requires the disposal ordischarge of the organic volatiles. Second, volatilization of thesolvent from the uncured resin can result in the presence of voids andirregularities in the prepreg and in the cured laminate. Furthermore, aconsiderable amount of time is required for the solvent removal step. Amethod for applying resin to the substrate which did not requiresolvents would thus have environmental, quality and efficiencyadvantages.

Processes for applying liquid-form resins to the substrate includepassing the substrate through a resin bath, as illustrated in U.S. Pat.No. 4,767,643, and coating a non-porous release sheet with liquid resinand then pressing the release film against the porous substrate totransfer the resin thereto, as described in U.S. Pat. No. 4,139,591. Theformer technique suffers from problems associated with the tendency ofresin in the reservoir to "advance," or partially cure, if it is notimmediately taken up by the substrate, and the latter method suffersfrom the inconvenience and expense of processing the release sheet. Itwould be desirable to develop techniques for resins application which donot involve the use of a resin bath or a release sheet.

It is therefore an object of the invention to provide a process andapparatus for impregnating a fibrous substrate with a thermosettableresin. In a specific aspect, it is an object of the invention to providea process and apparatus for impregnation of a fibrous glass substratewith a solventless thermosettable resin system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of the preferred embodiment of theinvention resin application apparatus.

FIG. 2 is a schematic drawing of a resin delivery system designed foruse with the invention process.

FIG. 3 is a flow diagram of a prepregging process incorporating theinvention resin application apparatus.

SUMMARY OF THE INVENTION

According to the invention, a process is provided for impregnating afibrous substrate with a thermosettable resin, the process comprising:providing resin application means comprising a moving surface;positioning on the moving surface a liquid-form thermosettable resinformulation comprising an essentially uncured thermosettable resin;passing a porous web in countercurrent contact with the thermosettableresin formulation on the moving surface so as to transfer thethermosettable resin formulation into the fibrous web; and passing theresin-containing fibrous web to a heating zone to partially cure theresin and form a prepreg comprising the fibrous substrate and thepartially-cured thermosettable resin.

Further according to the invention, apparatus is provided forimpregnating a porous web with a thermosettable resin, the apparatusincluding resin application means comprising a movable surface; meansfor applying a liquid-form thermosettable resin onto the movablesurface; means for advancing, in a countercurrent direction with respectto the direction of movement of the movable surface, a porous web to themovable surface and in contact with the thermosettable resin thereon andthence to a resin cure zone; and means not in contact with the oppositeside of the porous web at the point of resin transfer for maintainingtension in the web sufficient to promote transfer of the liquid resinfilm from the movable surface into the interior of the web.

DETAILED DESCRIPTION OF THE INVENTION

The invention process provides a technique for impregnating a fibroussubstrate with a liquid-form thermosettable resin. The process isparticularly suitable for impregnating a glass web with a solventlessresin system in the preparation of a prepreg for ultimate use inelectrical laminates.

The process involves contacting an advancing fibrous substrate with aliquid-form resin positioned on a moving surface and transferring, bycountercurrent contact, the resin from the moving surface to the fibroussubstrate and into the interior thereof. As used herein, such a fiberapplication process involves impregnating the resin into the interior ofthe fibrous substrate and is to be distinguished from coating only theexterior surface of the substrate. The object of the resin applicationprocess of the invention is to achieve thorough wet-out of the substrateby the liquid resin and to thereby permit the fabrication of ahigh-quality cured laminate from the resin-impregnated fibroussubstrate.

In the invention resin application process, a fibrous substrate isimpregnated with a liquid-form thermosettable resin. Although theinvention process can be practiced with solvent-borne resins, thepreferred resin system for the invention process is one which does notcontain an organic solvent, which includes both water-borne resinsystems and solventless resin systems. For a solventless resin system,the liquid form can be achieved by use of a thermosettable resin whichis a low-viscosity liquid at room temperature or which has been heatedto a temperature effective to achieve sufficiently low viscosity forthorough wetout of the substrate. In the latter case, of course, theresin system (the thermosettable resin and any curing compounds usedtherewith) must not cure to any substantial degree at its meltingtemperature over the length of time of the substrate impregnationprocess.

The invention resin application process and apparatus can be describedby reference to FIG. 1. The substrate 2 in web form, generally anyporous material in chopped, mat or woven form, preferably a web of wovenglass fibers, is advanced from delivery means 1, which will generallyinclude automatic means for advancing the web at a selected rate andwith a selected web tension. The fibrous web is optionally heated by,for example, infrared heaters, prior to advancement to the resinapplication zone. Guiding means 3 is positioned to direct the web towardresin applicator roll 4 at a predetermined arc of contact α. Angle α canvary depending upon the overall configuration of the application scheme,but will generally be within the range of about 20 to about 90 degrees,preferably about 20 to about 40 degrees, most preferably about 25 to 34degrees in the applicator embodiment shown. Resin application means 4,which can be, for example, an endless belt or a roller and is shown hereas a roller rotating in a counterclockwise direction, delivers liquidresin film 5 to a first surface of web 2 passing counterdirectionalthereto. Applicator roll 4 is maintained at a temperature effective tokeep resin film 5 in essentially uncured, liquid form. This temperaturewill vary depending upon the resin, but will generally be within therange of about 50° to about 200° C. The speed of rotation of applicatorroll 4, the tension in web 2 as it contacts resin film 5, and the speedat which web 2 is advanced to the applicator roll are coordinated toprovide good wetout of the web. These specifications can vary widelydepending, for example, upon the resin system, the type of web material,and the heating capacity of the downstream B-staging unit. In general,the speed of rotation of applicator roll 4 will be within the range ofabout 70 to about 125 percent of web speed, preferably about 90 to about100 percent of web speed; the tension in web 2 will generally be withinthe range of about 1 to about 3 pounds per linear inch, preferably about1.5 to about 2 PLI; and the speed of advancement of the web through theresin application zone will be within the range of about 8 ft/min toabout 200 ft/min, preferably 50 to about 150 ft/min.

Resin film 5 is applied to applicator roll 4 by means of resin deliverymeans, shown here as a combination of set gap roll 8 and nozzle 7capable of applying a controlled quantity of liquid resin to therotating surface of the applicator roll. Nozzle 7, represented here incross-section, can be a tube the axis of which is parallel to the lengthof the roll, having one or more exit ports for application of the liquidresin system to the roll. Blade 9, located closely adjacent the area ofresin application to roll 4 and in contact with set gap roll 8, can beused to prevent movement of newly-deposited resin on the surface ofroller 8 as it rotates and to define a small well or bead of activeresin in the set gap area. Nozzle 7 can be associated with any means forcontinuous delivery of the resin in liquid form, at either ambient orelevated temperature. Delivery of the resin will be carried out atvolume rates synchronized with the speed of the moving web so as todeliver a predetermined volume of resin to the web and to minimizeresidence time within the resin delivery system. Resin delivery meanscan include, for example, a temperature-controlled static blender or amixing extruder with an outlet into nozzle 7.

Metering means, shown here as a set gap roller 8 located between nozzle7 and the point of contact of resin film 5 and the advancing web, isused, in conjunction with resin delivery means 7, to control the amountof liquid resin which is delivered to the web. Set gap roll 8 ispreferably a smaller-diameter roll than applicator roll 4 so as topermit a generally vertical alignment of the set gap roll and theapplicator roll and to minimize residence time of the liquid resin priorto application to the web. Control of the rate at which resin is appliedto the web is achieved in the first instance by careful setting of thegap between set gap roll 8 and applicator roll 4 so as to maintain auniform film thickness 5. Secondly, the rotational speed of theapplicator roll is coordinated with web speed so as to achieve transferof the resin film onto the moving web. In addition, because no backuproll is employed to control the contact of the moving web with theapplicator roll, control of web tension is maintained to ensure stableoperation of the resin application process.

Resin removal means 10, shown here as a scraper blade located abovediscard trough 11, serves to remove from the applicator roll any resinwhich remains on the roll after contact between resin film 5 andadvancing web 2, as any resin which has remained on the roll for acomplete rotation is likely to have undergone excessive cure forapplication to the web.

Resin-containing web 6 is advanced to optional second resin applicationmeans 12, shown here as a roller rotating in a direction counter to thatof first roller 4. Second roller 12 can be used to smooth the secondsurface of the resin-impregnated web and can, if required, serve theoptional function of applying additional liquid-form resin 11 to the webas desired to increase prepreg resin content. This second roller canoptionally be heated. In the embodiment shown, liquid resin, thequantity of which is controlled by nozzle 15 and set gap roll 13, isapplied to resin-containing web 6 to increase the resin content of theweb and to fill in any interstices or voids in the resin-containing web.Roller 12 can serve the additional (or alternate, if no resin is appliedto the web at this point) function of smoothing the resin on the surfaceof the web as the web is advanced toward cure zone 14. It is preferableto pass the resin-saturated web directly to the cure zone withoutcooling thereof by cooling means such as chilled rollers, for example.

Second application means 12 is positioned between first applicationmeans 4 and cure zone 14. The web travel distance between theapplicators can vary depending upon the other process variables,including the resin formulation, web porosity and web speed, but thisdistance will generally be within the range of about 1 to about 5 feet,preferably about 1 to about 3 feet. The resin application process iscarried out in the absence of pressure applied to the web opposite thearea of resin transfer from the applicator to the web. In certainconventional resin application processes, such pressure is applied by abackup roll contacting the web surface (or a release sheet in contactwith the web surface) opposite the surface through which resin is beingapplied. In the invention process, such applicator backup rolls are notnecessary. The use of a release sheet is an option with the single-rollembodiment of the invention process, but is not necessary or, ingeneral, desirable in the practice thereof.

The wetout of the web by the resin is achieved partially bypressure-driven flow at the applicator roll 4, but mainly by capillaryaction within the web. The capillary-induced flow of resin into the webdepends upon the viscosity and surface tension of the resin as well asthe porosity of the web and surface tension of the web fibers. Forimpregnation of conventional glass web styles, resin formulations havingviscosities within the range of about 0.5 to about 6 poise, typicallyabout one poise, and resin surface tension within the range of about 25to about 40, typically about 32 dynes/cm, can be used. Resin infusiontimes will vary from about 0.1 second to 0.5 second under typicalconditions. For a web speed of 200 ft/min (65 m/min), for example, theweb will move approximately 1.6 ft (0.5 m) in 0.5 seconds. Therefore, toensure complete wetting of the web before it reaches the secondapplicator roll 12, at web speeds of the order of 200 ft/min, the secondapplicator roll 12 will typically be positioned so that the points ofweb contact at applicator rollers 4 and 12 are approximately 2 ft (0.7m) apart. Of course, modification of the process conditions and resincharacteristics will require modification of the applicatorspecifications, including the distance between applicator rolls.

Resin delivery means designed for use in the invention resin applicationprocess and apparatus can be described with reference to FIG. 2. Resinfrom reservoir 20 maintained, by optional temperature control means 21,at a predetermined viscosity is passed via conduit 22, pump 23 andconduit 24 to mixing means 31, shown here as a static blender withinternal blending baffles. Control system 29 delivers the desiredproportion of resin and curing agent to the mixer, which is coordinatedwith the line speed of the web through the resin application zone byvariable speed electric drive system 37. Curing compound(s) in reservoir25 maintained, by optional temperature control means 26, in liquid formare passed via conduit 27, pump 28 and conduit 30 to mixing means 31,wherein the resin component and the curing component are intimatelymixed at a controlled temperature maintained by temperature controlmeans 32. The mixed resin formulation passes via nozzle 33 to applicatorroll 35 and set gap roll 34, shown here with associated temperaturecontrol means 36, of the resin application apparatus described above.

The prepregging process of the invention can be described in generalterms by reference to FIG. 3. Fibrous web 43 is delivered to resinapplication zone 44 by a suitable automated web advancement system 42with means for measuring and controlling advancement speed and webtension. Web tension control devices are known in the art. For example,unwind roll 41 can include a brake which, in combination with afront-end dancer roll, maintains a preset web tension programmed into apull-in unit located between the dancer roll and the resin applicationzone. Similarly, proper downstream web tension can be maintained by adancer roll which moderates the speed of a variable-speedconstant-diameter roll located downstream from the heating zone.

The fibrous web is advanced through resin application zone 44, which ishere shown in the preferred generally vertical orientation, in which theweb passes in a generally upward direction as liquid resin is applied bythe method described in detail above. Application zone 44 includes resindelivery means, including a mixing portion for blending the resin andcuring system, and temperature control as necessary to maintain theresin system at the desired viscosity.

Resin-saturated web 45 is advanced from the resin application zone toresin cure zone 46, typically a forced air heated treater, wherein theresin-saturated web is treated, by exposure to elevated temperature orUV radiation, for example, to partially cure the resin without gelation,a process known as "B-staging." The temperature in the treatment zonewill vary depending upon the resin system and the degree of resin curedesired, but will generally be within the range of about 80°to about200° C., preferably about 120° to about 180° C. The resin-saturated webwill be subjected to the B-staging treatment for a time sufficient toimpart the desired degree of cure, generally about 30 seconds to about 8minutes. The web is advanced from resin treatment zone 46 in the form ofa prepreg 47, which is rolled at uptake roll 48 for storage or,alternatively, is passed directly to lamination.

A laminate is fabricated by subjecting a set of layered prepregs toconditions effective to cure the resin and to integrate the prepregsinto a laminated structure. The laminate can optionally include one ormore layers of a conductive material such as copper. Laminatingconditions generally include a time of about 30 minutes to about 4hours, preferably about 1 hour to about 2 hours, a temperature of about160° C. to about 300° C., preferably about 170° C. to about 200° C. anda pressure of about 50 to about 500 psi. The laminate can optionally be"post-cured" by heating at a temperature of about 200° to about 230° C.at ambient pressure for about 1 to 6 hours to improve thermalproperties.

Thermosettable resins which can be used in preparing electricallaminates include epoxy resins, imide resins, cyanate resins, propargylethers, and blends and reaction products thereof. The currently favoredresins, because of their low cost and cured properties, are epoxyresins, alone or blended with another resin. Suitable epoxy resins forelectrical laminates include glycidyl ethers of dihydric and polyhydricphenols. Exemplary diepoxy resins include those which are prepared bythe base-catalyzed reaction of a bisphenol and an epichlorohydrin andcan be represented by formula I: ##STR1## in which n is zero or a numbergreater than zero, commonly in the range of 0 to 10, preferably in therange of 0 to 2, and R is methylene or 2,2-propylene. An example of asuitable epoxy resin component is EPON® Resin 1123, a brominateddiglycidyl ether of bisphenol-A having a molecular weight of about 800.Also suitable as the epoxy resin component are multifunctional glycidylethers of the tetraphenol of ethane, as represented below in structureII. Such multifunctional epoxy ##STR2## resins are availablecommercially as EPON® Resin 1031 from Shell Chemical Company. Othersuitable resins can be prepared by the reaction of epichlorohydrin withmononuclear di- and trihydroxy phenolic compounds such as resorcinol andphloroglucinol, selected polynuclear polyhydroxy phenolic compounds suchas bis(p-hydroxyphenyl)methane and 4,4'-dihydroxybiphenyl, or aliphaticpolyols such a 1,4-butanediol and glycerol.

The epoxy resin component of the composite can also be novolac-basedepoxy resins ("novolac epoxy resins"), which are the glycidyl ethers ofthe product of reacting a phenol, such as phenol, cresol, resorcinol orbisphenol-A, with formaldehyde in acid solution. An example of asuitable class of bisphenol-A novolac epoxy resins is represented belowin structure III. ##STR3##

Other thermosettable resins, alone and in combination with epoxy resins,can be processed into laminates by the invention process. Suchthermosettable resins include, for example, cyanate esters, propargylethers, and vinyl esters, and blends of such resins with epoxy resins.Highly suitable thermosettable resins for electrical applicationsinclude imides such as bismaleimides and trismaleimides. Preferredbismaleimides include N,N'-bisimides of unsaturated carboxylic acidswhich can be represented by the formula ##STR4## in which Y is asubstituted or unsubstituted divalent radical containing at least 2carbon atoms, preferably 2 to 6 carbon atoms, and a carbon-carbon doublebond, and Z is a divalent radical containing at least 1, generally about1 to about 40 carbon atoms. Z can be aliphatic, cycloaliphatic, aromaticor heterocyclic. A preferred class of bismaleimides is derived from anaromatic diamine and can be represented by the formula ##STR5## in whicheach R₁ is selected independently from H, C₁₋₂ alkyl or halide; R₂ isselected from divalent hydrocarbon radicals containing from about 1 toabout 6 carbon atoms, --O--, --SO₂ --, --COO--, --CONH--, --CO-- and--S--S--; and each R₃ is selected independently from H, C₁₋₃ alkyl andhalide.

Examples of such bismaleimides include

N,N'-4,4'-methylene-bismaleimide

N,N'-4,4'-ethylene-bismaleimide

N,N'-hexamethylene-bismaleimide

N,N'-meta-phenylene-bismaleimide

N,N'-p-phenylene-bismaleimide

N,N'-4,4'-diphenylmethane bismaleimide

N,N'-4,4'-diphenylether bismaleimide

N,N'-4,4'-diphenylsulphone bismaleimide

N,N'-4,4'-dicyclohexylmethane bismaleimide

N,N -4,4'-(3,5-diphenylpyridine) bismaleimide

N,N -pyridinidi-2,6-Y bismaleimide

N,N -α,α'-4,4'-dimethylenecyclohexane bismaleimide

N,N -meta-xylelene bismaleimide

N,N -4,4'-diphenylcyclohexane bismaleimide

N,N -meta-phenylene bisdichloromaleimide

N,N -4,4'-diphenylmethane biscitraconimide

N,N -4,4'-(1,1-diphenylpropane) bismaleimide

N,N'-4,4'-(1,1,1-triphenylethane) bismaleimide

N,N'-4,4'-triphenylmethane bismaleimide

N,N'-3,5-(1,2,4-triazole) bismaleimide,

and various N,N'-bismaleimides disclosed in U.S. Pat. Nos. 3,562,223,4,211,860 and 4,211,861. Bismaleimides can be prepared by methods knownin the art, such as described in U.S. Pat. No. 3,018,290, for example.The imide can also be a trifunctional maleimide reaction product of abis(aminobenzyl)aniline with maleic anhydride. For laminatingapplications, the imide will preferably be blended with an epoxy resinin an amount within the weight ratios of 1:9 to 9:1, preferably about1:1 to about 9:1.

An epoxy resin-containing laminating composition will include a curingagent. Effective curing agents for epoxy resins are known to include,for example, amines, acids, anhydrides, phenols and imidazoles. Thepresently-preferred curing agents for imparting optimum laminatingproperties to epoxy compositions are phenolic compounds which have aphenolic functionality greater than about 1.75. The preferred phenoliccuring agents are phenolic novolacs prepared by reacting a dihydroxyphenol -such as resorcinol or bisphenol-A with formaldehyde in acidsolution. The preferred phenolic novolac resin curing agents arebisphenol-A novolacs having a weight per phenolic group of about 60 toabout 500, preferably about 60 to about 300, and, on the average, morethan about 2 phenolic hydroxyl groups per molecule, preferably about 3to about 5. Such phenolic novolacs are available under the tradenameEpikure® DX-175 from Shell International Chemical Company. The phenolicnovolac curing agent will be present in the composition in an amounteffective to cure the epoxy resin, which will generally be astoichiometric amount of about 0.75 to about 1.25 equivalents perequivalent of epoxy resin. In terms of weight percent, the curing agentwill be present in an amount generally from about 10 to about 70 weightpercent, preferably about 15 to about 50, most preferably about 15 toabout 40, based on the combined weight of epoxy resin and curing agent.

The curing agent, for flame-proof applications, can be a mixture of thephenolic resin curing agent and a brominated phenolic curing agent. Thebrominated phenolic curing agent can be any monomeric or polymericcompound having at least one free phenolic hydroxyl group and one ormore bromine atoms on the aromatic ring. Examples of suitable brominatedphenolic curing agents include brominated bisphenol-A novolac,brominated phenolic novolac, brominated polyphenylene oxide, brominatedbisphenol-A and brominated bisphenol-A carbonate, for example. Thebrominated bisphenol-A will be present in an amount effective toincrease flame retardancy, generally an amount up to about 40 weightpercent, usually about 2 to about 15 weight percent, based on thecombined weight of epoxy resin and curing agent(s).

In order to promote faster and/or lower temperature cure of the resincomponents of the composition, an optional cure accelerator may be used.Many suitable accelerators, such as ureas, tertiary amines, imidazoles,phosphenes, octoates and boron trifluorides, for example, are known inthe art. The presently preferred class is imidazoles such as 1-methylimidazole, 2-ethyl imidazole, 2-methyl imidazole, 2-methyl-4-ethylimidazole and isopropyl imidazole. Because of its availability andperformance characteristics, 2-methyl imidazole is the preferredaccelerator. The accelerator will be present in the composition in anamount effective to increase the cure rate and/or lower the curetemperature of the composition, generally in an amount from about 0.01to about 7, preferably from about 0.05 to about 3 weight percent, basedon the weight of the composition.

The thermosettable resin system must be designed within certainspecifications dictated by the resin application process parameters. Theresin formulation must be a liquid at a temperature at which the resindoes not undergo cure over the time necessary for application of theresin to the substrate. The resin system must be of sufficiently lowviscosity that it achieves good "wetout," or saturation of the web,without the use of a backup roll at the point of application. Onceapplied to the substrate, however, the resin system must have sufficientviscosity that it does not drop from the resin-containing web before itreaches the heating zone. Resin formulations having viscosities in therange of about 1 to about 10 poise, preferably about 1 to about 6 poise,are most suitable. The currently preferred resin system is a blend of adiglycidyl ether of bisphenol-A having a WPE of about 175-185, abrominated diglycidyl ether of bisphenol-A having a WPE of about 310-350and a bromine content of about 30-50%, a phenolic novolac curing agent,and 2-methylimidazole accelerator.

The process of the invention can optionally be practiced with athermosettable resin formulation which includes an organic solvent ordiluent present in an amount effective to decrease the viscosity of thesystem for ease of processing. Polar organic solvents such as ketones,alcohols and glycol ethers, for example, are suitable. The chosensolvent will generally have a boiling point less than about 160° C. Thepreferred solvents for epoxy resins are ketones such as acetone, methylethyl ketone and methyl isobutyl ketone, for example, and solventmixtures of these with an alkylene glycol ether such as propylene glycolmonomethyl ether. The proportion of solid components in the compositioncan vary widely depending upon the amount of the other constituentspresent and the intended application of the composition, but the solventin a solvent-borne system will generally constitute from about 15 toabout 50 weight percent of the total weight of the formulation.

EXAMPLE 1

This example describes the preparation and testing of a solventlesstheremosettable resin system to determine its suitability for use in theinvention prepreg preparation process. The resin component was preparedby charging 41.003 g brominated diglycidyl ether of bisphenol-A and58.997 g liquid diglycidyl ether of bisphenol-A (WPE 178-186) to aheating vessel and stirring under nitrogen at 120° C. for 30 minutes.The curing agent component was prepared by heating 98.271 g of aphenol-formaldehyde novolac (HRJ-1166 from Schenectady Chemicals, WPP103-105) to 120° C., adding 1.729 g of 2-methyl imidazole, and mixing at120° C. for 30 minutes. The resin formulation was prepared by blendingthe resin component and the curing agent component in a weight ratio of72:28.

A laminate was prepared under laboratory conditions simulatingconditions of resin delivery and application to a glass web to confirmthat the viscosity and cure characteristics of the resin formulation metprocessing requirements for the invention resin application technique.The viscosity of the above blend at 100° C. was about 9 poise. The blendwas mixed at this temperature for 3 seconds to simulate mixing in anextruder barrel. The resulting blend had a gel time (measured at 140°C.) of 89 seconds and a viscosity at 120° C. of 6 poise. After mixing at120° C. for 3 seconds to simulate one-side web application, the gel time(140° C.) was 85 seconds. The blend was applied by a handsqueeze-rolling technique to woven glass (style BFG 7628) and partiallycured in an oven for 2 minutes at 163° C. for simulation of aconventional treater oven B-staging operation. The dust gel of theB-staged resin (140° C.) was 40 seconds, confirming advancement of theresin.

A prepreg produced in this manner was made into an eight-ply laminate bypressing at a pressure of 200 psi and holding at a temperature of 347°F. Heat-up and cool-down rate for this operation were maintained at 25°F./min. The laminate had the following properties:

    ______________________________________                                        Flexural strength (23° C., psi)                                                                  72,000                                              Flexural modulus (23° C., psi)                                                                   3,550,000                                           Tg (°C., DSC)      152                                                 Dielectric constant (23° C., D-24/23)                                                            4.47                                                Volume resistivity (× 10.sup.13 ohm-cm)                                                           290                                                 Water absorption (1 hr 15 psi steam, % w)                                                               0.22                                                Copper peel (1 oz, Cu, lbs/in)                                                                          8.7                                                 Flammability UL-94        VO                                                  ______________________________________                                    

EXAMPLE 2

This example demonstrates application of a solventless resin system to aglass web using the invention process on a laboratory scale. Asingle-side coater was constructed using an adjustable experimentalcoating apparatus. The applicator roll diameter was 8 inches. A 1-inchMeyer rod was used to define a gap through which the resin was meteredprior to contact with the web. The applicator roll was rotated at asurface speed of 8 ft/min, and the Meyer rod was rotated at 1 ft/min.The applicator roll surface was heated to 120° C. A flexible, porouswoven glass fabric web (1080 glass style) 18 inches in width was fed tothe roller at a rate of 10 ft/min. The resin formulation described inExample 1 was deposited onto the applicator roll using a hot-meltdispensing pump operating at 100° C. Resin was applied to the cloth toprovide a resin content of 67% wt. The resin-containing web was passedthrough a 15-foot oven maintained at 140° C. to B-stage the resin. Thedust gel (measured at 171° C.) for B-staged resin removed from theprepreg was 34 seconds. The prepreg was used to make a 16-ply laminateby pressing at the following conditions:

    ______________________________________                                        rate of rise (deg F./min)                                                                            5                                                      maximum temperature (°F.)                                                                     347                                                    pressure (psi)         100                                                    time at temperature (hrs)                                                                            1                                                      cooldown rate (°F./min)                                                                       40                                                     The laminate had the following properties:                                    Flexural strength      53,271                                                 Flexural modulus       2,098,000                                              Tg                     135                                                    Volume resistivity     240                                                    Laminate appearance    clear, uniform,                                                               good adhesion                                                                 to glass, some voids                                   ______________________________________                                    

We claim:
 1. A process for preparing a prepreg comprising aglass-reinforced thermosettable resin, the process comprising:(a)depositing a liquid-form, solventless thermosettable resin formulationcomprising an essentially uncured epoxy resin onto the surface of arotating roller; (b) passing a porous glass web having first and secondweb surfaces in countercurrent contact with said resin formulation onsaid rotating ft/min and a web tension within the range of about 1 toabout 3 pounds roller at a web speed within the range of about 8 ft/minto about 200 per linear inch so as to transfer said resin formulation tothe first surface of the glass web and thence to the interior thereof,said transfer being carried out in the absence of external pressureapplied to the second web surface opposite the area of resin transfer tothe first web surface; and (c) passing said resin-containing glass webto a cure zone and subjecting the web to conditions effective topartially cure the thermosettable resin.
 2. The process of claim 1 inwhich the thermosettable resin formulation is deposited onto saidrotating roller from nozzle means extending from a static mixer.
 3. Theprocess of claim 1 in which the thermosettable resin formulation isdeposited onto said rotating roller from nozzle means extending from anextruder.
 4. The process of claim 1 in which said rotating roller ismaintained at a temperature within the range of about 50° to about 200°C.
 5. The process of claim 1 which comprises heating the glass web priorto contact thereof with the thermosettable resin formulation.
 6. Theprocess of claim 1 which further comprises:(a) providing a second rollerin general upward vertical alignment with respect to said first roller,said second roller rotating in reverse direction with respect to saidfirst roller; (b) passing said resin-containing glass web from saidfirst roller to said second roller so as to effect countercurrentcontact between the second surface of the resin-containing glass web andthe second roller, said contact effected in the absence of externalpressure applied to the first web surface opposite the area of contactwith said second surface.
 7. The process of claim 6 which furthercomprises depositing onto the surface of said second roller aliquid-form, solventless thermosettable resin formulation comprising anepoxy resin and passing the second surface of said resin-containingglass web in countercurrent contact therewith so as to transfer thethermosettable resin thereto.
 8. The process of claim 1 in which step(b) comprises forming the deposited resin into a film of meteredthickness for contact with the porous glass web.
 9. The process of claim1 in which said rotating roller is rotating at a speed within the rangeof about 70 to about 125 percent of the speed of the web.
 10. Theprocess of claim 1 in which the porous glass web is passed in contactwith the moving surface at a speed within the range of about 50 ft/minto about 150 ft/min.
 11. The process of claim 1 in which the epoxy resincomprises a diglycidyl ether of bisphenol-A having a weight per epoxidewithin the range of about 175-185.
 12. The process of claim 1 in whichthe thermosettable resin formulation further comprises at least one ofan imide resin, a cyanate ester resin and a propargyl ether resin. 13.The process of claim 1 which further comprises passing the epoxy resinand a curing agent into a mixing chamber wherein said thermosettableresin and said curing agent are blended at an elevated temperature toform the liquid-form thermosettable resin formulation.
 14. The processof claim 1 in which the rotating roller is maintained at a temperatureeffective to maintain the viscosity of the thermosettable resin withinthe range of about 0.5 to about 6 poise.
 15. The process of claim 1 inwhich the web speed is within the range of about 50 to about 150 ft/min,the web tension is within the range of about 1.5 to about 2 pounds perlinear inch, and the speed of rotation of the roller is within the rangeof about 90 to about 100 percent of the web speed.
 16. The process ofclaim 1 which further comprises passing the resin-containing glass webto a second rotating roller and causing sufficient countercurrentcontact between said second roller and the second surface of theresin-saturated web to smooth said second surface and to remove anyexcess resin therefrom.